Aaron Schaffhausen Update: Wis.father pleads guilty but insane in daughters' deaths

(CBS/AP) HUDSON, Wis. - A Wisconsin father accused of killing his three young daughters last summer changed his not guilty plea to guilty but insane Thursday.

PICTURES: 3 Wisconsin sisters dead; father arrested

Aaron Schaffhausen, 35, switched his plea after more than a day of delays and legal wrangling about what kind of evidence will be allowed at his trial, which starts next week.

With the plea change, prosecutors won't have to prove Schaffhausen killed his daughters and or that he tried to set fire to their River Falls home in July 2012.

But the defense will have to prove by a preponderance of the evidence that Schaffhausen had a mental disease or defect, and that he lacked substantial capacity to appreciate that what he did was wrong or couldn't control his impulses.

Police found Schaffhausen's daughters, 11-year-old Amara, 8-year-old Sophie and 5-year-old Cecilia, dead in their beds. Their throats were slit, and gasoline was poured in the basement. He is charged with three counts of first-degree intentional homicide and one count of attempted arson.

Prosecutors alleged that Schaffhausen killed his daughters to get back at his ex-wife, Jessica, because he was bitter over their divorce and angry because he thought she began seeing another man.

According to the criminal complaint, Schaffhausen texted his ex-wife on July 10, 2012 to ask for an unscheduled visit with the girls, and she consented. Schaffhausen arrived and sent the babysitter away.

About two hours later, he called his ex-wife and, according to the complaint, told her: "You can come home now because I killed the kids."

Jury selection is to begin Monday in St. Croix County Circuit Court.

Complete coverage of Aaron Schaffhausen on Crimesider